3 Answers
3

In OpenLayers, the base layer dictates what is going to be the "base spatial reference". So any overlay will be projected to the base srid. Since any base layer that comes from the consumer-based providers (google, bing, yahoo, etc) supports only 900913, then OpenLayers is doing the right thing for your.

modify the WMS server definition to make sure that 900913 is allowed as a valid reprojection

add a definition for that new srid (depends on what web server you are using)

a combination of thse

By looking at the URL, it seems you are using an ASP .NET WMS server, since Geoserver is java and Mapserver if cgi, and ArcGIS Server is .NET, I am going guess this is an ESRI server, so the instructions on how to modify this is here.

Update: As mkennedy points out, the official EPSG code for SphericalMercator (aka Google's projection) is 3857 and a previous assigned number was 3785. If you are wondering why there are so many numbers that refer to the same spatial reference, crschmidt's blog post will give you a clue...

That request is correct - Google uses a spherical mercator projection (epsg:900913) so if you request your WMS layer in 3785 it won't line up with your Google base map. If your server is unable to reproject the data (as appears to be the case) you can set up a cascading WMS (both GeoServer and MapServer can do this) to fetch the WMS layer in 3785 and reproject it to 900913 for you.